Those who suffer from depression are well-acquainted with the urge to stay in bed all day and sob for no reason other than the fact that attempting to dam the chemically imbalanced reservoir of sadness within you is about as plausible as watching just one episode of Scandal in a sitting.

But there’s another symptom people with depression and bipolar disorder deal with—one that’s hard to pin down but has finally found some scientific backing: "Fuzzy thinking" or "brain fogginess."

Whether it involves difficulty concentrating or simply not thinking clearly, brain fogginess can be frustrating for people with mental illness, in part because there's no clinical diagnosis for brain fog. But according to new research published in the journal Brain, the symptom is linked to fundamental differences in the brain activity of women with depression and bipolar disorder.

In the study, researchers from the University of Michigan gave a simple cognitive test to 612 women—over two thirds of which were living with either major depression or bipolar disorder. The women were told to watch a sequence of letters flash on a screen in front of them. When certain letters popped up, they were asked to “react rapidly.”

While many women with mood disorders had scored as well as women without mood disorders, nearly all the women who performed the worst (bottom 5 percent) had depression or bipolar.

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The researchers also arranged for 52 of the participants to undergo brain scans while performing the test and found that women with mood disorders showed different levels of activity in the right posterior parietal cortex (PPC), the part of the brain that controls executive functions like visual and spatial processing—as well as attention and memory. Women with depression showed higher levels of right PPC activity compared to women with no mood disorder, while those with bipolar showed lower levels.

The big takeaway? The fact that women with both types of mood disorders both experienced the common symptom of fogginess—and both showed abnormal levels of brain activity—led the researchers to suggest the way we diagnose mental health disorders may need rethinking.

"In all, we show a shared cognitive dysfunction in women with mood disorders, which were pronounced in the cognitive control tests and more nuanced in scans," Kelly Ryan, lead author and clinical assistant professor of psychiatry, said in a press release.

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Ryan points out that while, traditionally, psychiatric diagnoses have focused on distinguishing between mood disorders, the current system may not be the most effective. Researchers are "not finding huge differences between what clinicians see as categories of disease," she said. "This raises questions about traditional diagnoses."

For patients and their psychiatrists, a mental health disorder diagnosis based on subjective experiences can be a tricky process—but understanding the neurobiology behind mood disorders could make diagnosis more efficient.

If there’s one thing more frustrating than getting all lugubrious over spilled milk (or spilled almond milk, in my case) because that’s just how your brain works, it’s dealing with the fact that your brain doesn’t function like it used to or you feel like it should. But knowing there’s an actual neurological explanation for such cognitive dysfunction provides some solace—and the potential for a more effective treatment is even more comforting.